In conjunction with non-metallic inclusions contained in a metal, the present invention relates to an evaluation apparatus for cleanliness of a metal, and a method therefor, which quickly discharges non-metallic inclusions contained in a steel, for example, to the surface portion, detects the non-metallic inclusions accumulating the surface either chemically or physically, and accurately determines the proportion of the non-metallic inclusions in the metal as a total quantity evaluation or as an evaluation of principal components in accordance with a particle size distribution.
Hereinafter, the explanation will be given using steel as a typical example of a metal. Non-metallic inclusion particles existing in the steel include alumina type inclusions formed as the result of the reaction between oxygen in the steel and aluminum added in the case of an aluminum killed steel, slag type inclusions containing lime/silica, etc., and resulting from a steel making slag, powder type inclusions resulting from a casting mold lubricant in continuous casting, and so forth. Since these inclusions result in defects such as flows and breakage in intermediate products, during rolling of thin sheets, wire materials, etc., or in final products, evaluation of these inclusions by various methods have bean carried out in the past for the purpose of quality control.
If any defects are found in the final product, on the other hand, it is a serious problem to discard the product at the final stage from the aspect of the production cost because the product is produced through various production steps. It is therefore desirable to evaluate quality at an early stage of the production. Particularly because the existence of the inclusions is determined at the stage of refining/solidification of the metal, various evaluation technologies have been conducted in the past.
The evaluation technology of the inclusions of the steel among the metals is described, for example, in xe2x80x9cSteel Handbook, 3rd Editionxe2x80x9d, II Pig Iron and Steel Making (edited by Japan Iron and Steel Institute of Japan, published by Maruzen, Oct. 15, 1979). Examples of the evaluation methods include a total oxygen (T[O]) method based on the oxygen concentration in the steel, a slime method by electrolytic extraction used for evaluating large inclusions, a microscopic method for evaluating the inclusions by magnifying and observing the section of a metal, and so forth. Due to their respective features, these technologies are limited by the kind of inclusions as the investigation object and the sizes of the inclusions as tabulated in Table 1, and they are not free from the problem, either, that a long time is necessary depending on the evaluation method.
It is known that information of intermediate products is not sufficient so as to estimate the product defects. In other words, as shown in Table 1, the conventional means involves the problems that the evaluation sample does not sufficiently represent the quality of the intermediate product and a long time is necessary for the evaluation of the sample, and those methods which invite excessively great super-heat during melting such as an EB (electron beam melting) method involve the problem that the inclusions are denatured during evaluation.
The slime method has been widely employed as a method having relatively high accuracy, but an extremely long time of several days to dozens of days is necessary to electrolyze about 1 kg sample as a whole.
When the evaluation is made by a small amount of metal sample, a metal piece sample of a part of large amounts of metal is evaluated. Therefore, to strictly evaluate the cleanliness of the whole metal, a large number of samples must be collected from the same metal piece, and the problem to be solved is to speed up the evaluation of the cleanliness.
On the other hand, though the melting means is different from the EB method, an induction melting extraction method using a cold crucible method is conceivable as the same melting extraction method. In other words, this method eliminates the problems such as high temperature melting of the EB method and the resulting modification of the inclusions, and insufficiency as the representative value by the evaluation volume of the small amount. A method of measuring the inclusions of the surface of the sample produced by this cold crucible levitation-melting method is described, for example, in xe2x80x9cEvaluation of Alloy Cleanness in Superclean Materialsxe2x80x9d, K. C. Mills et al., Turkdogan Symposium Proceedings, pp. 105-112 (1994). The method of this reference inspects the surface inclusions by a scanning electron microscope. However, this reference points out only the problem as the evaluation method by the characteristics of the cold crucible itself, but does not teach the method of evaluating the non-metallic inclusions over a wide area of the metal surface industrially, economically and quickly.
FIGS. 1(a) and 1(b) are explanatory views of the principal portions of a cold crucible apparatus, wherein FIG. 1(a) is an explanatory plan view, and FIG. 1(b) is an explanatory view of the longitudinal section taken along Axe2x80x94A of FIG. 1(a). In FIG. 1, reference numerals (1-1, . . . , 1-8) denote eight, for example, copper segments which together form a crucible and the inside of which is cooled with water. They are disposed adjacent to one another with the gap slits 3 interposed at a plurality of substantially equidistant positions and form the crucible. Reference numeral 2 in the drawings denotes an induction coil, which is so disposed as to encompass the crucible.
FIGS. 2(a) and 2(b) are explanatory views of the operation of the cold crucible. When a high frequency current flows through the induction coil 2 in a direction indicated by an arrow 5, an inducted electromotive force occurs in a direction indicated by an arrow 6-1 occurs on the side of the induction coil 2 of the segments 1. Since the segments 1 are spaced apart from one another by the slits 3, however, the induction current does not flow through other adjacent segments, but flows as an induction current in a direction indicated by an arrow 6-2 on the opposite side to the induction coil. Reference numeral 4 in the drawing represents the metal sample. An eddy current flows through the metal sample 4 in a direction indicated by an arrow 7 due to the induction current in a direction indicated by an arrow 6-2. The metal piece 4 is heated by the eddy current in the direction of the arrow 7 and is melted. In this instance, since the eddy current flows through the molten metal 4 in the direction of the arrow 7, repulsion 8 acts in the center direction of the metal due to the induction current in the direction of the arrow 6-2 that flows through the segments 1, and this repulsion 8 keeps the molten metal 4, under a levitating and non-contact state, away from the segments 1.
The cold crucible method melts the metal sample, due to levitation, in a non-oxidizing atmosphere and holds the levitating molten metal. During this retention time, the non-metallic inclusions in the metal sample are discharged to the surface of the molten metal as indicated by reference numeral 9 in FIG. 2(b). When the current to be passed through the coil is cut off after retention for a predetermined time, the molten metal is solidified while the non-metallic inclusions gather on the surface thereof. The cleanliness of the metal piece is evaluated by measuring the non-metallic inclusions gathered on the surface of the solidified body.
According to the prior art method which measures the non-metallic inclusions scattered inside the metal piece, measurement is complicated and requires a long measurement time but according to the cold crucible method, the measurement of the non-metallic inclusions gathering on the surface can be easily made because they gather on the surface of the solidified body and, moreover, within a short time. According to the prior art method which measures the non-metallic inclusions contained and scattered in the metal, the sample is extremely small, and is not correct as a representative value of the steel. On the other hand, because the cold crucible method can levitate several grams to several kilograms of the metal sample, the quantity of the sample is greater than before, and evaluation can be made more correctly over a typical values of the steel.
If the quality of intermediate products corresponding to quality of products of metal pieces can be quickly evaluated as compared to the prior art, the production cost and time can be drastically improved. The present invention is completed on the basis of this concept.
In other words, the present invention is directed to solve the problems of representativity of the evaluation samples in quality evaluation of the intermediate products, the problems of the measurement time and cost, and the problem of denaturing of inclusions. If the cold crucible treatment alone is merely carried out and the non-metallic inclusions are merely gathered on the sample surface, it takes a long time to investigate the surface by using the microscope and to count the number of the non-metallic inclusions, as described in the reference described above, and the intended objects cannot be accomplished.
To accomplish the objects described above, the present invention provides an apparatus, and a method therefor, which can gather non-metallic inclusions to the most advantageous position for the measurement of the whole quantity by a cold crucible, and can efficiently measure the whole quantity.
The gist of the present invention resides in the following points.
(1) An evaluation apparatus for cleanliness of a metal, comprising: metal levitation-melting means which comprises a water-cooled metal crucible including a bottom surface having a curvature and a sidewall surface having a sloped surface gradually expanding upward, and having slits interposed in a radial direction, an induction coil for generating a repulsion from the sidewall surface of the water-cooled metal crucible to a center direction, and passing a high frequency current for melting the metal while levitating the metal, and a container for maintaining a non-oxidizing atmosphere; handling means for taking out a metal having non-metallic inclusions accumulating at a specific position on the surface of the metal melted and solidified inside the metal levitation-melting means, and transferring the metal to analyzing means; and the analyzing means for analyzing the non-metallic inclusions so accumulated.
(2) An evaluation apparatus for cleanliness of a metal according to the item (1), comprising: metal levitation-melting means which comprises a water-cooled metal crucible comprising a plurality of segments divided in a circumferential direction, and having an open upper surface and a closed lower surface, an induction coil for passing a high frequency current, disposed in such a manner as to encompass the water-cooled metal crucible, and a non-oxidizing atmosphere container; handling means for taking out a metal melted and solidified by the levitation-melting means from the water-cooled metal crucible, moving the metal, and capable of setting the metal to a predetermined analysis position; and energy dispersion type fluorescent X-ray means for analyzing non-metallic inclusions accumulating on the surface of the metal.
(3) An evaluation apparatus for cleanliness of a metal according to the item (1), comprising: metal levitation-melting means which comprises a water-cooled metal crucible comprising a plurality of segments divided in a circumferential direction, and having an open upper surface and a closed lower surface, an induction coil for passing a high frequency current, disposed in such a manner as to encompass the water-cooled metal crucible, and a non-oxidizing atmosphere container; metal transferring means for taking out a metal melted and solidified by the levitation-melting means from the water-cooled metal crucible, and transferring the metal to predetermined processing means; and acid-dissolving or electrolyzing means for extracting non-metallic inclusions concentrated on the surface of the metal melted and solidified by the processing means.
(4) An evaluation apparatus for cleanliness of a metal according to the item (1), comprising: metal-levitation means which comprises a water-cooled metal crucible comprising a plurality of segments divided in a circumferential direction, and having an open upper surface and a closed lower surface, an induction coil for passing a high frequency three-phase alternating current for imparting a repulsion moving upward on the surface of a molten metal along the wall of the crucible while levitating and melting the metal thereinside, disposed in such a manner as to encompass the water-cooled metal crucible; and luminance difference/area conversion means for analyzing non-metallic inclusions accumulating on the upper surface of the metal melted and solidified by the levitation-melting means.
(5) An evaluation apparatus for cleanliness of a metal according to the item (1), wherein means for supplying a current to be passed through the induction coil is a single-phase alternating current source.
(6) An evaluation apparatus for cleanliness of a metal according to any of the items (2) through (4), wherein the shape of the inner surface of the crucible has a shape formed by cutting a rotating body having the symmetry axis of a perpendicular axis into halves on a plane of symmetry, and a shape formed by an upper shape of a circular truncated cone having the same shape as that of the symmetry plane or an upwardly expanded similar shape of the horizontal section.
(7) An evaluation apparatus for cleanliness of a metal according to any of the items (2) through (4), wherein the bottom surface of the crucible is shaped in such a manner that the bottom of the inner surface in an area of at least 90% by the diameter of the inner surface becomes a flat surface.
(8) An evaluation method for cleanliness of a metal comprising the steps of: levitation-melting a metal piece for a predetermined time by using levitation-melting means; discharging non-metallic inclusions contained in the metal piece to the surface of a molten metal; and directly analyzing a curved and non-smooth surface of the metal after solidification by a fluorescent X-ray analysis means using an energy dispersion type spectroscope so as to measure quantities of elements constituting the non-metallic inclusions and to identify the quantity of the non-metallic inclusions.
(9) An evaluation method for cleanliness of a metal according to the item (8), comprising the steps of: levitation-melting a metal piece for a predetermined time by using levitation-melting means; discharging non-metallic inclusions contained in the metal piece to the surface of a molten metal; rotating either intermittently or continuously the metal having a curved and non-smooth surface round an axis connecting the uppermost point and the lowermost point at the time of melting as the center thereof; directly analyzing the surface of the metal by a fluorescent X-ray analysis means using an energy dispersion type spectroscope; measuring the quantities of elements constituting the non-metallic inclusions; and identifying the quantities of the non-metallic inclusions in accordance with the kind or the origin.
(10) An evaluation method for cleanliness of a metal comprising the steps of: levitation-melting a metal piece for a predetermined time by using levitation-melting means; discharging non-metallic inclusions contained in the metal piece to the surface of a molten metal; dissolving the surface of the metal after solidification by an acidic solution or electrolyzing it in an aqueous type solution or a non-aqueous type solution; extracting and filtrating the non-metallic inclusions; and weighing and analyzing the non-metallic inclusions so filtrated, or weighing and analyzing them after separation.
(11) An evaluation method for cleanliness of a metal according to the item (10), wherein the retention time t (seconds) of the levitation-melted metal for accumulating the non-metallic inclusions contained in the metal to the surface of the levitation-melted metal falls within the following range (1):
1xe2x89xa6t/{square root over ( )} (30 d)xe2x89xa620xe2x80x83xe2x80x83(1)
where d is a maximum inner diameter (mm) of the crucible.
(12) An evaluation method for cleanliness of a metal characterized in that measurement of non-metallic inclusions accumulating on the surface of the top of a molten metal is carried out by the steps of cutting off a high frequency current after a metal sample is levitation-melted, the difference of luminance between the surface of the metal sample during cooling and the non-metallic inclusions is photographed by a CCD camera, and island-like occupying areas of the non-metallic inclusions are measured by image processing the image so photographed.
(13) An evaluation method for cleanliness of a metal according to the item (11), comprising the steps of: carrying out a levitation-melting treatment by changing t/{square root over ( )} (30 d) (t: retention time of a levitation-melted metal (seconds), d: maximum inner diameter (mm) of crucible); determining in advance the relation between t/{square root over ( )} (30 d) and a diameter L of the non-metallic inclusions by investigating the diameter L occurring at maximum frequency at each t/{square root over ( )} (30 d) value; selecting a desired value for t/{square root over ( )} (30 d) when the cleanliness of another metal is evaluated, and carrying out the levitation-melting treatment for the other metal; measuring the occurring quantity N of the non-metallic inclusions having the diameter L in the other metal by estimating that the diameter L of the non-metallic inclusions occurring at the maximum frequency in the other metal at this selected t/{square root over ( )} (30 d) value is the same as the relation that is determined in advance; and evaluating this N as cleanliness of the other metal.
(14) An evaluation method for cleanliness of a metal according to the item (13), wherein the occurring quantities N1, N2, . . . of the non-metallic inclusions having diameters L1, L2, . . . greater than L are measured in the other metal, and the N1, L1, . . . values are evaluated as cleanliness of the other metal.
(15) An evaluation method for cleanliness of a metal according to the item (10), wherein at least 10 particles are selected from particles having the maximum diameter in the non-metallic inclusions discharged, and the diameters of the non-metallic inclusions having the maximum particle diameters existing in the metal from which the metal piece is collected, are estimated by a statistical extremes method.